clk: sunxi-ng: Prevent unbinding CCUs via sysfs

The CCU drivers are not really designed to be unbound. Unbinding a SoC's
main CCU is especially pointless, as very few of the peripherals on the
SoC will work without it. Let's avoid any potential problems by removing
the bind/unbind attributes from sysfs for these drivers.

This change is not applied to the "secondary" CCUs (DE, USB) as those
could reasonably be unbound without making the system useless.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210901050526.45673-3-samuel@sholland.org